The present invention relates generally to an objective optical system, and more specifically to an objective optical system having a focusing function. For instance, the invention relates to an endoscope objective lens capable of enlarged-scale proximity viewing, or a taking lens for digital or video cameras capable of microphotography, commercially marketable small-format cameras, and so on.
In the medical endoscope field, there has recently been mounting demand for an optical system capable of implementing enlarged-scale closeup viewing for close examinations of lesions. Such an enlarged-scale view endoscope objective lens is broadly broken down into two types: one comprising +−+ three groups wherein the negative, second group moves for focusing on a near point, as set forth in Patent Publications 1, 2 and 3, and another comprising −+− three groups wherein the positive, second group moves for focusing, as disclosed in Patent Publication 4.
Allowing endoscopic images to have even higher quality has also been demanded so as to improve the precision of diagnosis, and so imaging devices having a lot more pixels than ever before are now being used. There is thus demand for high-performance imaging devices more compatible with imaging devices having an increasing number of pixels.
For endoscopic diagnosis of biotissues such as lesions, on the other hand, there has been mounting demand for optical systems capable of implementing ultra-enlarged-scale closeup (microscopic) viewing as much as microscopes can. An endoscope objective optical system allowing such endoscopic viewing to be extended to microscopic viewing of cells or the like is set forth in Patent Publications 5 to 8, etc.
So far for microscopic viewing of biotissues, there has been no option but to identify a lesion by endoscopic viewing and, thereafter, take some cells out of it by biopsy for microscopic viewing. However, the above proposals have the merit of providing an extension of endoscopic viewing to in vivo cell viewing.
In the commercial products field including digital cameras, video cameras, etc., too, much is still left to be desired in terms of the high magnifications and size reductions of optical systems for macrophotography or the like.    Patent Publication 1: JP(B) 61-44283    Patent Publication 2: JP(A) 6-317744    Patent Publication 3: JP(A) 11-316339    Patent Publication 4: JP(A) 2000-267002    Patent Publication 5: JP(A) 2004-313769    Patent Publication 6: JP(A) 2004-313772    Patent Publication 7: JP(A) 2007-233036    Patent Publication 8: JP(A) 2007-260305
With the optical systems set forth in Patent Publications 2 to 4 or 7 of the above prior arts, magnifications at the time of enlarged-scale closest range viewing are not high enough for microscopic viewing.
Patent Publication 1 comes up with an enlarged-scale, high-magnification endoscope object lens; however, it has some practical problems, because the field of view upon normal viewing remains narrow, there is the need of implementing in vivo screening to find lesions, much difficulty is encountered in getting on with the affected site, and so on. This endoscope objective lens is also less than satisfactory in terms of resolution of objects upon enlarged-scale viewing because its optical performance is not that good.
The optical system for microscopic viewing set forth in Patent Publications 5 and 6 is provided separately of an optical system that implements normal endoscopic viewing, because it has fixed magnification and is only capable of implementing enlarged-scale closeup viewing.
In practical microscopic viewing, an optical module for microscopic viewing must be guided to a subject via a scope attachments insertion channel. For this reason, there is parallax occurring upon switching endoscopic viewing over to microscopic viewing, and so much difficulty is experienced in identifying the range of where to view. Further, it is difficult to fix the optical system for microscopic viewing to the site being viewed, because its diameter is small. In other words, those who can handle this scope capable of microscopic viewing are limited to a skilled expert or physician alone.
To allow the operator to have a sense of security in manipulating the scope during microscopic viewing, two optical systems may possibly be laid out in a single endoscope apart from the attachments insertion channel. However, this is nothing else but a single unit with two scopes combined in it, and gives rise to outer diameter increases in particular, which would make the burden too heavy for patients.
In consideration of the above problems, some of Patent Publication 7 or Patent Publication 8 discloses an optical system designed such that viewing from normal endoscopic viewing to enlarged-scale closeup microscopic view can be implemented with a single optical system. The optical system set forth in Patent Publication 7 is less than satisfactory for microscopic viewing, because the maximum magnification at the nearest point is a bit lower. When it comes to monitor viewing, electronic enlargement is needed to obtain sufficient magnifications; although there is a high viewing magnification obtained, yet the resolution of images remains low, often resulting in image deterioration.
The optical system disclosed in Patent Publication 8 is satisfactory in terms of the maximum magnification, but focusing is implemented by movement of two groups. For this reason, there is the need of mounting a driving mechanism of implicated construction on it, which is a factor of size increases. In addition, lens control is difficult to gain because of quite independent movement of moving lens groups.